The present invention relates to method for geophysical prospecting by radioactive diagraphy, and to apparatus for the execution of these methods.
Various methods and pieces of equipment for geophysical prospecting by radioactive diagraphy are known, in which a probe comprising a radioactive source is displaced vertically in a drill-hole, the said radioactive source bombarding the surrounding terrain, and one or more particle detectors which detect the particles emitted by the terrain, thus making it possible to collect information relating to the nature thereof.
The said radioactive sources may be pulsed neutron generators, the functioning of which may be periodically interrupted in order to measure the delayed neutrons or gamma-rays emitted by the isotopes originating in the terrain as a result of the neutron bombardment.
However, pulsed neutron generators are costly and cumbersome.
The radioactive sources may also be radioactive elements, for example a fragment of californium 252. In this case, it has already been suggested that the delayed particles be measured by periodically moving the said source away from the detector within the probe, while the probe is being displaced in a continuous movement. This method makes it possible to use small amounts of radioactive elements which may be accommodated in small-diameter probes which are particularly suitable for drill-holes used in mining research.
It is to be understood that the radioactive sources may be sources of neutrons or gamma-rays at any level of energy, and that the particle detectors may be of any type known for detecting either gamma-rays (photons), or neutrons, or both.
Known methods of radioactive diagraphy, whereby the radioactive source is periodically displaced within the probe in order to vary the vertical distance separating the source from the detector, in the case of a single detector, or separating the said source from each of the detectors, in the case of detectors located on each side of the source, have disadvantages since, while the delayed radiations are being measured, the detector is not located precisely opposite the terrain which has just been bombarded, but at a distance therefrom equal to the minimal thickness of the screen separating the source from the detector, which is of the order of 30 to 40 cm. This distance reduces the sensitivity of the apparatus and makes it impossible to detect isotopes having very short lives.
It is an aspect of the present invention to perfect known methods and apparatus, in order to overcome this disadvantage, and to make it possible, while retaining the continuous displacement of the probe, to measure the induced radioactivity immediately after the end of the bombardment, the detector being located opposite the zone which has just been bombarded by the source.
Accordingly, this invention provides a geophysical prospecting method using radioactive diagraphy in a drill-hole, said method comprising the steps:
Vertically displacing in the said drill-hole a probe comprising a radioactive source and at least one particle detector separated by a screen, PA1 simultaneously displacing said source within the probe, for the purpose of periodically varying the vertical distance separating the source from said at least one detector, PA1 while the source is remote from said at least one detector, using the detector to measure the delayed radiations, having a specific energy, which are emitted by the terrain bombarded by the source during the time when the latter was close to the said detector, PA1 and periodically displacing said at least one detector vertically within the tube in synchronism with the said source.
The purpose of the invention is achieved by displacing each detector periodically and vertically within the probe, in synchronism with the source.
Each detector is preferably displaced vertically in the same direction as the source and by a distance less than that by which the source is displaced.
Each detector may, with advantage, be displaced by means of a reciprocating motion of an amplitude substantially equal to the minimal distance separating the source from the said detector when they are close together.
A probe according to the invention may be associated with electronic measuring circuits comprising a precision analyzer which makes it possible to count pulses due solely to well-defined energy particles.
A diagraphy probe according to the invention may be displaced in continuous motion and at a certain speed which is independent of the life of the isotopes under study. The probe may also be displaced intermittently, always in the same direction, by halting the movement of the probe while the particle source is remote from the detector.
In both cases a semi-continuous diagraph of a sounding is obtained. The diagraph may be taken while the probe is descending. In this case the particle source is located below the detector, and the probe comprises a downward extension which allows the source to be moved away from the detector.
The diagraph may also be taken while the probe is ascending. In this case the detector is located below the source and the probe comprises an upward extension making it possible to move the source away from the detector. This second solution has the advantage of making it possible to obtain a diagraph extending to the bottom of the drill-hole.
The following description relates to the drawings attached hereto and illustrating, in no way restrictively, examples of embodiment of the invention.